This invention deals with the field of solid film lubricants, more particularly the solid film lubricants used on booster rocket components and associated launch apparatus.
Multi-body launch vehicles require the use of solid film lubricants (SFLs) to allow for unrestricted relative motion between structural assemblies and components during lift-off and ascent into orbit. They are also used on fasteners to provide precise torque-tension relationships during clamping and also de-torquing. The solid film lubricants used for these purposes must provide adequate lubricity under conditions of extremely high load for a relatively short duration. In addition to providing lubrication under extremely high load conditions, the SFL must not promote corrosion. This factor is of major importance since booster vehicles and the apparatus associated with launching them are exposed to the corrosive action of sea air mist and fog prior to launch because the launch sites are generally situated in close proximity to the sea. As these vehicles are generally recovered from the sea after launch they are also exposed to the corrosive action of sea water.
To meet the requirements for lubricity, lead and antimony bearing compounds are commonly used. Although graphite is an effective lubricating component of SFL compositions, the tendency of graphite to create an electrolytic cell in the presence of sea water that results in corrosion of the metallic surface to which it has been applied, severely limits its use for this application. To avoid this problem, antimony compounds have been used instead of graphite in some SFL formulations. Cadmium plated fasteners are also in common use because of the corrosion protection and lubrication provided by a cadmium surface. However, when cadmium plated fasteners are used on aluminum alloys electrolytic corrosion results. Although technically useful in some SFL applications, cadmium, lead and antimony compounds are also highly toxic to humans and other animal life. In addition, the environmentally safe disposal of lead, cadmium and antimony derivatives presents a major problem from both the safety and economic points of view. The costs for disposal of these hazardous materials continues to escalate and adequate protection for humans while using or disposing of these materials borders on the impossible.
In current practice, a ceramic bonded basecoat solid film lubricant (SFL) that contains a high level of lead compounds and graphite is initially applied to the solid rocket booster hardware by spray coating and then cured at 1000.degree. F. in an oven. Prior to application the part must be masked. The coated part is then masked again and a lead containing silicone topcoat is then spray applied. The top-coated part is then subjected to a second curing operation at 500.degree. F. The process of applying and curing this dual coat system is extremely complicated, costly and also hazardous. Due to its lead content the ceramic bonded basecoat is hazardous to the health of those applying it, while the presence of graphite causes corrosion and pitting of the reusable solid rocket booster in the presence of sea water, thereby reducing its service life. The need to cure at 1000.degree. F. requires a high level of energy expenditure and special heating apparatus. The requirement for two coats and the precautions that must be taken to avoid poisoning the workers are costly in both materials and time. Furthermore, in the event of damage such as a break in the ceramic two coat SFL system, repair is difficult, costly and inconvenient.
There is, therefore, a need for a solid film lubricant composition for use on rocket boosters and associated launch apparatus as well as in other heavy machinery applications that can provide lubricity for moving parts under extremely high load conditions, while at the same time not promoting corrosion of the substrate metal. The SFL must, in addition, be environmentally safe while being relatively non-toxic to humans, be easy to apply, preferably in one coat, and require relatively low cure temperatures and be relatively inexpensive to manufacture and use. Furthermore, should the SFL become damaged, it should be easy to repair in an inexpensive safe and convenient manner.